Oocyte maturation in the amago salmon (Oncorhynchus rhodurus): In vitro effects of salmon gonadotropin, steroids, and cyanoketone (an inhibitor of 3β‐hydroxy‐Δ5‐steroid dehydrogenase)

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Abstract
The effect of partially purified chinook salmon gonadotropin (SG‐G100) and a number of steroids on the induction of germinal vesicle breakdown (GVBD) in amago salmon (Oncorhynchus rhodurus) oocytes (with intact follicle layers) was investigated in vitro. SG‐G100 was effective only at the highest concentration tested (1 μg/ml). 17α,20β‐Dihydroxy‐4‐pregnen‐3‐one (17α,20β‐diOHprog) was the most potent maturation‐inducing steroid tested, followed by 17α‐hydroxyprogesterone. Testosterone or deoxycorticosterone (DOC) enhanced the rate of GVBD in response to SG‐G100. DOC also enhanced the response to 17α,20β‐diOHprog but testosterone was without effect, suggesting that DOC has a direct action on the oocyte while testosterone probably acts at the level of the follicle. Estradiol‐17β had no effect on GVBD in response to SG‐G100 or 17α,20β‐diOHprog.The action of SG‐G100 was shown to be dependent on the synthesis of a second Δ4 steroidal mediator of maturation since cyanoketone, a specific inhibitor of 3β‐hydroxy‐Δ5‐steroid dehydrogenase, completely abolished the maturational effects of the gonadotropin and pregnenolone but not Δ4 steroids. Radioimmunoassay of media in which oocytes were induced to mature in vitro with SG‐G100 revealed significantly elevated levels of progesterone and 17α,20β‐diOHprog. Estradiol‐17β levels, high in control media, were only elevated twofold by SG‐G100. Levels of the two progestogens were extremely low or nondetectable in media in which oocytes were incubated with cyanoketone, while estradiol‐17β levels remained high. These results are discussed in relation to other evidence indicating that 17α,20β‐diOHprog is the naturally occurring maturation‐inducing steroid of amago salmon. The role of other steroid hormones, particularly the possible involvement of corticosteroids, in the control of final oocyte maturation in teleosts is explored.